The present invention relates to a nasal mask adapted to be used in carbon dioxide gas measurement, and more particularly, used with an apparatus for a CPAP (Continuous Positive Airway Pressure) therapy or an NPPV (Noninvasive Positive Pressure Ventilation) therapy that is used for treatments for patients suffering from sleep apnea syndromes or respiratory failure.
In the treatments, a nasal mask is fixed to tightly contact an area around a nose for preventing leakage from a gap therebetween and pressure inside the nasal mask is increased by using gas such as oxygen or air that is transmitted from the apparatus described above. Although the object of the CPAP therapy or the NPPV therapy is to improve the ventilation state of patients, the enhancement of the concentration of carbon dioxide gas accumulated in the patients' bodies due to hypoventilation can be acquired quantitatively by simultaneously measuring carbon dioxide gas in the expired gas. To be more specific, there is a desirable value for the concentration of carbon dioxide gas, and a difference between an actual concentration of carbon dioxide gas and the desirable value can be recognized at a glance. In addition, an optimal level of the positive pressure or an optimal number of respirations in the NPPV therapy can be set in a simple manner by acquiring the concentration of carbon dioxide gas in the expired gas. Further, by continuously monitoring the concentration of carbon dioxide gas in the expired gas, it is possible to immediately detect accidents in connection with abnormalities of respiratory circuit of the patient, respiratory condition of the patient, and measurement equipments, and to notify the same to a medical staff.
Japanese Patent Publication No. 2003-315264A discloses a configuration for measuring carbon dioxide gas, in which the expired gas is guided to a sensor by inserting nasal tubes in nostrils. However, since a sensor body is positioned right below the nostrils, contaminations such as nasal discharge are adhered on the sensor body along with the nasal tubes, and whereby the precision of the measurement is lowered. Accordingly, the frequency of cleaning and sterilization of the sensor should be increased or the sensor should be replaced in a short period. In addition, when oxygen or air that has been humidified is supplied to the inside of the mask, the frequency of cleaning and sterilization of the sensor should be increased further more. In addition, when the carbon dioxide gas sensor is included inside the mask, a gap between a cable and the mask is formed to cause the occurrence of a leak, and the efficiency for generating a positive pressure is decreased, and in a worst case, the positive pressure cannot be generated.
U.S. Pat. No. 7,004,168B2 discloses a configuration that measures carbon dioxide gas by extracting the expired gas together with sending oxygen to the mask. However, in this structure, precise measurement of concentration of carbon dioxide gas cannot be performed since the expired gas and oxygen supplied from the outside are mixed inside the mask and the expired gas is diluted by the supplied oxygen. In addition, since the inside of the mask is opened to the air, it is difficult to maintain a positive pressure inside the mask even by sending oxygen or the like forcibly.
Japanese Patent Publication No. 2002-520104T discloses a configuration in which a carbon dioxide gas sensor is attached outside the mask. In this case, since an end of the sensor which is not attached to the mask is opened to the air, it is difficult to maintain a positive pressure inside the mask even by sending oxygen or the like forcibly.